ORIGIN OF ZIRCON RIMS AROUND ILMENITE IN MAFIC PLUTONIC ROCKS OF PROTEROZOIC ANORTHOSITE SUITES

Abstract

Zircon rims around ilmenite occur in a wide variety of mafic plutonic rocks (Fe–Ti oxide ores, anorthosite, and gabbro) from unmetamorphosed Proterozoic intrusions in the Grenville Province of eastern Canada (Havre-Saint-Pierre, Mattawa and Saint- Urbain anorthosites, Quebec; Twin Lakes intrusive complex, Ontario) and the western U.S. (Laramie anorthosite, Wyoming). The rims range from a few μm to 100 μm across and occur along grain boundaries between Ti-based oxide minerals (mainly ilmenite and exsolved ferrian ilmenite, but also rutile) or between silicate minerals (plagioclase and biotite) and Ti-based oxide minerals. The zircon rims contain very low concentrations of P (<447 ppm), U (0.9–1.1 ppm) and Th (15–19 ppm), which, coupled with the restriction of the rims to ilmenite grain boundaries, rules out formation from high-temperature, zircon-saturated interstitial liquid and from low-temperature hydrothermal fluids. There is no textural evidence that zircon replaced pre-existing baddeleyite that may have formed by an oxidation–exsolution mechanism from ilmenite. The textural and compositional characteristics of the zircon rims are consistent with an origin by diffusion of Zr from ilmenite to grain boundaries during slow cooling and subsequent reaction with available Si from adjacent silicate minerals (or from ilmenite itself) to form zircon. A positive correlation between whole-rock TiO2 and Zr contents in samples of Fe–Ti oxide ores from Saint-Urbain and Big Island (Havre-Saint-Pierre) (up to 500 ppm in massive ilmenite ore) confirms that Zr is compatible in ilmenite crystallized from ferrobasaltic magmas. Mass-balance constraints demonstrate that the observed quantities of zircon can be derived from Zr-bearing ilmenite crystallized from basaltic magmas with typical Zr concentrations (161–252 ppm) after fractionation of Zr-free minerals (150–247 ppm). Zircon rims are not expected to occur in ilmenite-bearing basaltic lavas and high-level intrusions owing to rapid cooling, but zircon rims may be a common feature of slowly cooled plutonic rocks that crystallize from ferrobasaltic magmas. The U–Pb dating of these rims may provide additional constraints on the thermal evolution of a given plutonic rock.